Chronic exposure of solar ultraviolet (UV) radiation to human skin is more than primarily responsible for one million new cases of nonmelanoma skin cancer each year in the USA, making it the most prevalent environmental carcinogen known for humans. It is well documented that UV radiation is a potent producer of reactive oxygen species and can act as tumor initiator and promoter. UV-induced oxidative stress plays a crucial role in activation of cellular signal transduction pathways which are implicated in various skin diseases including cancer. There have been concerted efforts to identify newer and effective chemopreventive agents that can prevent UV carcinogenesis in humans. In earlier studies, we have shown that silymarin, a plant flavonoid from milk thistle (Silybum marianum L.), has potent anti-photocarcinogenic and anti-oxidant properties in an animal model (Katiyar et al, JNCI, 89:556-566, 1997). The current proposal is designed to determine the mechanism of prevention and identification of the cellular targets responsible for the prevention of photocarcinogenesis by silymarin treatment in the mouse model. The hypothesis to be tested in this proposal is that UVB exposure to skin induces oxidative stress, which causes activation of epidermal growth factor receptor (EGFR) and mitogen-activated protein kinase (MAPK) signaling pathways leading to the induction of photocarcinogenic events. The corollary to this hypothesis is that topical treatment of silymarin to SKH-1 hairless mice will prevent UVB radiation induced oxidative stress, and thus oxidative stress-mediated activation of EGFR and MAPK signaling pathways which will result in reduction of skin cancer incidence. The following specific aims are proposed to test this hypothesis. In Specific Aim-l, experiments will be conducted to determine whether topical treatment of silymarin will prevent UVB radiation induced: (a) depletion of cutaneous antioxidant defense components, such as glutathione peroxidase, catalase and glutathione (GSH), and (b) markers of oxidative stress, such as release of H202, nitric oxide and lipid peroxidation. In Specific Aim-II, we will determine whether treatment of silymarin will prevent UVB-induced oxidative stress mediated phosphorylation of EGFR, and MAPK such as ERKI/2, JNK and p38 signaling pathways. The results obtained from this study would lead to the generation of new knowledge by which silymarin exert's its chemopreventive effects, and will identify new methods and strategies for the prevention of solar UV radiation-induced skin cancer in humans, which may result to protect and prolong the human health and lives.